1. Technical Field
The disclosed embodiments of the invention are related in general to hydraulically boosted brake systems, and in particular, to such systems in which high-pressure fluid flows in a power booster only while a braking event is in progress.
2. Description of the Related Art
Modern vehicles are generally provided with features to assist the driver in controlling the vehicle, such as power steering and power brakes. With regard to power brakes, a device senses the driver's pressure on the brake pedal and applies an increased pressure to the brakes of the vehicle, proportionate to the pressure applied by the driver. In vehicles that employ gasoline powered engines, vacuum from the engine is typically used in a booster device to amplify the pressure applied to the brake pedal. Diesel engines do not produce vacuum, so diesel powered vehicles employ one of a number of alternatives. In very large vehicles such as freight and delivery vehicles, a separate compressor is typically provided to produce air pressure or vacuum to power the brake booster. In passenger vehicles, a hydraulic booster is often used, which is driven by a hydraulic pump that also generally powers the power steering booster.
FIG. 1 shows components of a hydraulically boosted braking system 100 that includes a master cylinder 110, a brake fluid reservoir 108, and a partial cut-away view of a hydraulic power brake booster 102. The system also includes a hydraulic fluid reservoir 103, a hydraulic fluid pump 105, fluid transmission lines 107, and an electric auxiliary fluid pump 111. The vehicle brake pedal 106 is coupled to the brake booster 102 by a pushrod 130.
The brake booster 102 includes a housing 112 with inlet and outlet fluid ports 118, 120, respectively, and a cylinder bore 150. A power piston 114 is positioned within the cylinder bore 150 and includes a piston bore 115 and an output pushrod 117. The power piston 114 includes a seal 142 that contacts a wall of the cylinder bore 150 and separates a low-pressure side 119 of the cylinder bore from a high-pressure side 121 of the cylinder bore 150. The power piston 114 is provided with low-pressure-side fluid vents 134 and high-pressure-side fluid vents 132. A poppet valve 136, a valve return spring 137, a valve rod 116, a reaction piston 138, and a feedback spring 128 are positioned within the piston bore 115 of the power piston 114, and a valve rod retainer 146 forms a seal at an open end of the piston bore 115 through which the input pushrod 130 contacts the valve rod 116. A boot 144 covers and protects a portion of the power piston 114 that extends from the booster housing 112. A flow switch 122 is positioned in the casing 112 adjacent to the fluid outlet 120.
In operation, the pump 105 is coupled to a drive shaft of the engine of the vehicle, generally via the fan belt, and is in constant operation while the engine is running. As long as there is no pressure applied to the brake pedal 106, the power piston 114 is positioned at the far right of the cylinder bore 150, as viewed in FIG. 1. Pressurized fluid from the pump 105 enters the cylinder bore 150 via the fluid inlet 118 on the high-pressure side of the power piston 114. The fluid passes into the piston bore 115 via the high-pressure-side vents 132, flows past the poppet valve 136, and exits the piston bore 115 via the low-pressure-side vent 134 to the low-pressure side 119 of the cylinder bore 150. The fluid exits the cylinder bore 150 via the fluid outlet 120 and thence to the fluid reservoir 103.
When the driver applies pressure to the brake pedal 106, the pushrod 130 transmits the pressure to the valve rod 116, moving the valve rod to the left, compressing the feedback spring 128. When the end 140 of the valve rod 116 contacts the poppet valve 136, the poppet valve 136 also begins to move to the left into a narrowed portion of the piston bore 115, which begins to throttle the flow of fluid through the piston bore. As fluid flow is impeded, fluid pressure increases in the high-pressure side of the cylinder bore 150, which causes the power piston 114 to begin to move to the left. The movement of the power piston 114 is transmitted by the output pushrod 117 to a piston of the master cylinder 110, thereby applying boosted braking force to the brakes. Meanwhile, if the driver depresses the brake pedal 106 a short distance and then holds, the power piston 114 moves to the left away from the valve rod 116, which allows the poppet valve 136 to withdraw slightly from the narrowed portion of the piston bore 115 and permit more fluid to pass. When an equilibrium is reached between fluid pressure on the high-pressure side 121 of the cylinder bore and a combination resistance from the master cylinder and the increased bias of the return spring 126 on the low-pressure side 119 of the cylinder bore, the power piston stops moving and remains stationary until the driver either presses harder on the brake pedal 106 or releases pressure from the brake pedal.
When the brake is applied, and fluid pressure increases on the high-pressure side of the cylinder bore 150, the increased fluid pressure also bears against the leftmost side of the reaction piston 138, which, in response thereto, moves to the right, which increases the bias of the feedback spring 128, thereby applying a proportional feedback pressure to the brake pedal 106 via the pushrod 130. If the driver applies greater pressure to the pedal 106, the power piston 114 will move further to the left before the pressure in the high-pressure side of the cylinder bore 150 rises sufficiently to move the reaction piston 138 far enough to apply a feedback pressure equal to the input pressure. When the driver releases pressure from the brake pedal 106, the valve rod 116, biased by the feedback spring 128, moves to the right, allowing the poppet valve 136, biased by the valve return spring 137, to move to a fully open position, which in turn allows fluid to again flow freely through the piston bore 115 of the power piston 114, thereby permitting the power piston 114 to move to the right to its full limit of travel.
It can be seen that, except when the brake is applied with maximum pressure, thereby completely closing the poppet valve 136, there is a continuous flow of hydraulic fluid through the booster 102. In the event of a malfunction that results in a loss of fluid flow into the fluid inlet 118, the flow switch 122 senses the resulting drop in fluid flow from the fluid outlet 120, and activates the electric auxiliary pump 111, which begins circulating fluid through the cylinder bore 150 to provide backup boosting power to the brake booster 102.